This invention relates to air ventilation systems and, in particular, to an air flow control system which has minimum variance in air volume delivery.
Several problems are involved in maintaining a reasonably constant volume of air flow in any air supply system. The term "system" refers to the external physical installation to which the fan is connected and is any combination of air flow passages and apparatus which imposes resistance to the flow of the air. The system is usually comprised of duct work and connections, heaters, air washers and filters, economizers, preheaters, furnaces, etc., and the pressure required to force air through such a system is dependent upon the rate of volumetric flow. The system resistance is usually expressed in terms of the design volume in cubic feet per minute (cfm) and the particular static pressure in inches of water column (W.C.) required to maintain the flow at the design rate.
One problem involves the characteristics of the fan which results in a reduction of air volume for any increase in static pressure requirement.
Another problem of maintaining a reasonably constant volume of air flow in the air supply system is that the system components manifest a pressure drop differential proportional to the volume of air flowing through them. Some components of the system exhibit a pressure drop which approaches a linear proportion, while other components exhibit an exponential relationship. Most components as a rule of thumb are assumed to follow the square law, i.e., the pressure differential across them increases (or decreases) as the square of the increase or decrease in air volume.
Because of such varying proportionalities between pressure drop and volume of air flowing through the system components, the designer of an air supply system must select an operating point on the fan performance curve for a given rpm and then allow the air volume to fluctuate between an upper limit and a lower limit as the components, such as the filters, become loaded. The designer may decide to choose a point between such lower and upper limits as the desired operating point with a corresponding volume of air flow. In reality, the actual air volume will normally be above such operating point (and thus be too high) or below the selected operating point (and thus be too low). In larger air ventilation systems such variance can be substantial. For example, an air flow system designed as described above for 30,000 cfm and having a static pressure range of 1.5" W.C. may typically have an air flow swing of 5,000 cfm or more; which constitutes a variation of almost 17% in air volume.
While the air flow system design just described may be the most practical approach, in some applications such design approach is not acceptable and the variation in air volume must be maintained within closer tolerances. Elaborate and costly monitoring systems are utilized for keeping the air flow volume stabilized in air ventilating systems.
It is an object of the invention to provide an improved air ventilation control system which stabilizes the volume of air flow in a simpler manner and at lower cost than prior art systems.
Another object is to provide such an improved volume control ventilation system which introduces artificial variable resistance to the air flow that permits substantial reduction in air volume fluctuations in comparison to prior art systems.
It is another object of the invention to provide an improved air ventilation system which controls the air volume to allow the fan, or blower, differential pressure to operate within 0.1" W.C. or less. A further object is to provide such improved air flow control system wherein the air volume fluctuations are reduced as low as 5%, or even as low as 3%, of the nominal system capacity.
In indoor gun firing ranges it is important to maintain the air volume supplied by the air ventilation system within very close tolerances. Specifications for such indoor firing ranges require that the air velocity moving past the marksman must not be below 75 feet per minute and must not be allowed to increase sufficiently so that vortexes are created. It is a specific object of the invention to provide an air ventilation supply system wherein the minimum velocity of the air at the firing line of such indoor firing range is 75 feet per minute and the maximum variation in air volume from this criteria is not more than 5-10 feet per minute, thereby avoiding any problem of creation of vortexes.